Daily rhythms in many behavioral, physiological and biochemical functions are generated by endogenous oscillators that function as internal 24 hr clocks. While the suprachiasmatic nucleus (SCN) of the anterior hypothalamus is established as the site of mammalian circadian pacemaker(s), it is not yet well understood how different circadian rhythms are entrained with proper phase relationships which are essential for human health and well-being. Human sleep patterns and other daily rhythms are often disrupted by external influences like jet-lag, shift- work or sleeping pills or by changes in internal status such as blindness. Recent findings in mice indicate that there is a unique class of behavioral zeitgeber (time-giver), the presence and absence (PA) cycles of the mother that entrains the circadian rhythms of pups despite their eyes remaining closed (a situation similar to blindness). The mouse pups are totally dependent on the mother during the early postnatal period. The pups' rhythms are maintained with a proper phase relationship to the outside world through their mother's daily behavior. However, the exact mechanism by which the mother mouse entrains her offspring is not known. The long-term objectives of this project are to elucidate the mechanism and significance underlying the behavioral model of postnatal maternal entrainment. The experimental animals will be the inbred mouse strain C57BL/6. The specific aims are to test five hypotheses. They are: 1. There is maternal entrainment in C57BL/6 animals nd it acts by means of a circadian mechanism; 2. Temperature acts as an entraining signal; 3. Olfaction may play a role in maternal entrainment; 4. The possible involvement of ingestive cues in entraining pups' rhythm; and 5. Maternal entrainment will not occur after the time when young animals reach sexual maturation. These goals will be achieved by using the following methods: 1. by behavioral assay 2. by recording temperature and 3. by surgical ablation of the SCN. Knowledge to be obtained from this project will be useful in understanding the basic concepts and the factors influencing the regulation of circadian rhythms.